1. Field of the Invention
The present invention relates to telecommunications, and, in particular, to fault protection schemes for optical communication networks.
2. Description of the Related Art
FIG. 1 shows a block diagram of a portion of a passive optical network 100 comprising an optical subscriber unit (OSU) 102, a passive optical splitter 104, and two optical network terminals (ONTs)—ONT #1106 and ONT #2108. OSU 102 exchanges optical signals with splitter 104 via bi-directional optical fiber 110, while splitter 104 exchanges optical signals with ONT #1106 via bi-directional optical fiber 112 and with ONT #2108 via bi-directional optical fiber 114.
OSU 102 functions as a central hub that transmits downstream optical signals received from a backbone network to splitter 104, which copies and broadcasts the downstream optical signals to both ONT #1 and ONT #2. This broadcasting of downstream optical signals is represented in FIG. 1 by the transmission of a downstream optical signal comprising data packets VC1 and VC2 from OSU 102 to splitter 104 over fiber 110, which passively splits that downstream optical signal into two identical copies for transmission to ONT #1 over fiber 112 and to ONT #2 over fiber 114.
In the upstream direction, ONT #1 and ONT #2 transmit different upstream optical signals over fibers 112 and 114, respectively, to splitter 104, which passively combines and transmits the two upstream optical signals as a single combined upstream optical signal over fiber 110 to OSU 102, which routes the combined upstream optical signal to the backbone network.
In order to avoid the different upstream optical signals generated by the different ONTs from interfering with each other when they are passively combined at optical splitter 104, in one implementation of a passive optical network, the different upstream optical signals from the ONTs are combined based on the principles of time division multiple access (TDMA) multiplexing, in which each ONT is assigned a unique time slot in the combined (i.e., TDMA) upstream optical signal transmitted from splitter 104 to OSU 102. By transmitting information in discrete data packets and by accurately timing the transmission of those data packets from the various ONTs, the arrival of the various data packets at splitter 104 can be orchestrated to coincide with the corresponding time slots in the upstream TDMA optical signal transmitted from splitter 104 to OSU 102. In this way, the different upstream optical signals from the different ONTs can be passively combined by splitter 104 to generate the upstream TDMA optical signal without any interference between data packets from different ONTs. This TDMA multiplexing is represented in FIG. 1 by ONT #1 transmitting a data packet VC3 to splitter 104 via fiber 112 and ONT #2 transmitting a data packet VC4 to splitter 104 via fiber 114 with the timing of those transmissions coordinated such that splitter 104 can passively combine the two upstream optical signals in the optical domain to generate and transmit an upstream TDMA optical signal comprising data packet VC3 followed by data packet VC4 to OSU 102 over fiber 110.
In general, the distance from splitter 104 to each ONT may vary from ONT to ONT. As such, the time that it takes for an upstream optical signal to travel from ONT #1 to splitter 104 may differ from the time that it takes for an upstream optical signal to travel from ONT #2 to splitter 104. In order to ensure accurate TDMA multiplexing using a passive optical splitter, the round-trip duration for transmissions between splitter 104 and each ONT is characterized using a procedure called ranging. During ranging, OSU 102 transmits a special downstream message that causes ONT #1 to transmit a corresponding upstream acknowledgment message back to OSU 102. OSU 102 measures the total round-trip duration from the time of the transmission of the special downstream message until the time of the receipt of the corresponding upstream acknowledgment message from ONT #1. OSU 102 then repeats the same procedure with ONT #2 to measure the total round-trip duration for ONT #2. OSU 102 compares the different measured round-trip durations to generate timing offsets to be applied by the different ONTs when transmitting upstream data packets to splitter 104 to ensure accurate TDMA multiplexing.
Since each ONT may transmit at a different signal power level over an optical fiber having a different length and since optical splitter 104 passively combines the different upstream optical signals received from the different ONTs, the upstream TDMA optical signal that arrives at OSU 102 will, in general, consist of a sequence of data packets, where each data packet may have a different signal power level. In order for OSU 102 to be able to accurately route the different data packets to the backbone network, OSU 102 is configured with a special type of receiver called a burst mode receiver (BMR). One of the characteristics of a BMR is the ability to reset its trigger level (i.e., the threshold for distinguishing between 1s and 0s in a received optical signal) at the beginning of each different data packet in a TDMA optical signal in order to handle the different signal power levels of the different data packets.
FIGS. 2A-D show time lines that demonstrate the capability of a BMR to adjust its trigger level at the beginning of each data packet (i.e., burst). In particular, FIG. 2A shows a time line corresponding to a TDMA optical signal comprising a burst from an ONT w, followed by a burst from a different ONT x, followed by a burst from yet another ONT y, followed by a burst from still another ONT z, where each different burst from each different ONT happens to arrive at the BMR with a different signal power level.
FIG. 2B shows Case (a), where a fixed high trigger level, as shown in FIG. 2A, is used to process each received data packet. In this case, only the data packet from ONT y is accurately decoded. The data packets from the other ONTs are lost, because the high trigger level misinterpreted all of that data as 0s. Similarly, FIG. 2C shows Case (b), where a fixed middle trigger level, as shown in FIG. 2A, is used to process each received data packet. In this case, only the data packets from ONT y and ONT z are accurately decoded. Note that the case of applying a fixed low trigger level (not shown in FIG. 2) may result in 0s being misinterpreted as 1s for data packets having a high signal power level. FIG. 2D shows Case (c), where the BMR uses a variable trigger level, as shown in FIG. 2A, to process each data packet. In this case, the trigger level is dynamically adjusted at the start of each burst of data (i.e., each data packet) to accurately decode each data packet from each different ONT.
As in all telecommunication systems, in order to ensure survivability and restore services following the occurrence of various service-affecting defects, it is desirable to configure networks, such as passive optical network 100 of FIG. 1, with fault detection and protection switching capabilities. To ensure high reliability over a wide range of services, the network should be able to restore services very quickly, usually on the order of 60 to 200 ms. Protection switching functionality should ensure quick restoration of service. Additionally, it should support automatic detection of failures, generically supported by “health check” functions and other suitable protocols.